1. Field of the Invention
The present invention relates to a sheet feeding device for feeding a sheet. More particularly, the present invention relates to a sheet feeding device, which has a straight sheet feeding passage and plural sheet feeding sections, each of which feeds a sheet while curving the sheet so that the sheet enters the straight sheet feeding passage. In addition, the present invention also relates to an image forming apparatus, such as a copier, printer, facsimile machine, or the like, having the sheet feeding device.
2. Discussion of the Related Art
Conventionally, image forming apparatuses like that shown in FIG. 1 are known.
FIG. 1 is a schematic view illustrating the sheet feeding device of such related-art image forming apparatus. Referring to FIG. 1, it can be seen that the sheet feeding device is arranged at the bottom of the image forming apparatus, and has three sheet feeding sections, i.e., first, second, and third sheet feeding sections. The first sheet feeding section includes a first sheet cassette 901, which is detachably attachable to the chassis of the image forming apparatus, and a first roller unit 904A arranged on the right side of the first sheet cassette. Similarly, the second sheet feeding section includes a second sheet cassette 902, which is detachably attachable to the chassis of the image forming apparatus, and a second roller unit 904B arranged on the right side of the second sheet cassette. The third sheet feeding section includes a large-capacity sheet cassette 903, and a third roller unit 904C arranged on the right side of the large-capacity sheet cassette.
As illustrated in FIG. 1, the first and second sheet cassettes 901 and 902, each of which serves as a sheet container for containing sheets P, are arranged in such a manner that the second sheet cassette is located above the first sheet cassette. The large-capacity sheet cassette 903, which can contain a large number of sheets P, is arranged above the second sheet cassette 902. Each of the right side portions of the first, second, and third roller units 904A, 904B and 904C defines a portion of a vertical sheet feeding passage 920, through which the sheet P is vertically fed from the lower side thereof to the upper side thereof.
FIG. 2 is an enlarged view illustrating only the second roller unit 904B of the sheet feeding device illustrated in FIG. 1 among the three roller units. Referring to FIG. 2, the second roller unit 904B includes a pickup roller 902a, a feed roller 905B, a reverse roller 906B, a grip roller 907B, a first driven roller 908B, a second driven roller 909B, etc. The uppermost sheet of the sheets P contained in the second sheet cassette 902 is fed out by the rotated pickup roller 902a so as to enter a separation nip formed by the feed roller 905b and the reverse roller 906B. If multiple sheets are fed simultaneously from the sheet cassette 902, the sheets are separated at the separation nip so that only the uppermost sheet passes through the separation nip. The thus-fed sheet P then enters a first nip N1 formed by the grip roller 907B rotated counterclockwise by a driving device (not shown) and the first driven roller 908B, which is contacted to the grip roller so as to rotate while being driven by the grip roller. As illustrated in FIG. 2, that portion of the sheet feeding passage which extends from the pickup roller 902a to the first nip N1 is slightly slanted upward in the sheet feeding direction.
Not only the first driven roller 908B but also the second driven roller 909B is contacted to the grip roller 907B, although at a different position from that of the first driven roller 908B. The second driven roller 909B is contacted to the grip roller 907B from the right side to form a second nip N2. The second nip N2 forms a portion of the vertical sheet feeding passage 920, which substantially vertically feeds the sheet P from the lower side thereof to the upper side thereof. The sheet P passing through the first nip N1 formed by the grip roller 907B and the first driven roller 908B and entering the vertical sheet feeding passage 920 strikes the second driven roller 909B. The sheet P is then sharply curved by the clockwise rotating second driven roller 909B so as to face upward and enter the second nip N2 formed by the grip roller 907B and the second driven roller. The sheet P is then fed upward through the vertical sheet feeding passage 920.
The second roller unit 904B has been described by reference to FIG. 2. The first and third roller units 904A and 904C have substantially the same configuration as the second roller unit 904B.
Although capable of handling ordinary sheets of paper or the like, the above-described configuration can have problems handling thicker media because such sheets are naturally stiffer, more rigid, and less easy to bend or curve. Thus, when a thick sheet having high stiffness is used as the sheet P in a sheet feeding device having such configuration as mentioned above, the sheet cannot be sharply curved by the second driven roller, often resulting in jamming of the sheet at the entrance of the vertical sheet feeding passage. For example, in the second roller unit 904B illustrated in FIG. 2, it is hard to sharply curve a thick sheet, which passes through the first nip N1 formed by the grip roller 907B and the first driven roller 908B and strikes the second driven roller 909B, so that the sheet faces upward in the vertical sheet feeding passage 920. Therefore, the sheet jamming problem tends to arise at the entrance to the second nip N2.
In order to prevent occurrence of the above-described sheet jamming problem, it is to be possible to feed such a thick sheet, which has passed through the first nip N1, along a curved passage having a large curvature radius so that the sheet changes the feeding direction little by little. However, since such a curved passage occupies much space, the image forming apparatus would become unacceptably large.
Hereinbefore, the sheet jamming problem caused in a sheet feeding device in which a sheet fed from a sheet cassette is curved so as to face upward in a vertical sheet feeding passage has been described. Such a jamming problem is caused not only in such a sheet feeding device, but also in sheet feeding devices in which a sheet fed from one of plural sheet cassettes is curved soon after entering a straight sheet feeding passage so as to be fed to the downstream side in the straight sheet feeding passage.
For these reasons, there is a need for a sheet feeding device which can properly curve a thick sheet fed from a sheet cassette at an entrance of a straight sheet feeding passage to securely feed the thick sheet to the downstream side in the straight sheet feeding passage without causing the sheet jamming problem.